The present invention relates generally to telecommunications networks, and more particularly to a method and system for enabling number portability in IMS networks.
Traditional telecommunications such as public switched telephone network are circuit-switched networks whereby end-to-end communication is facilitated by uniquely identifying an origination node and a termination node. The origination nodes and the termination nodes are tied to the physical locations of the two networks in which the nodes reside, and the identification numbers such as the telephone numbers corresponding to the nodes are generally not portable. More specifically, a telephone number corresponding to a node connected to a local telephone exchange cannot be ported to another node connected to a different telephone exchange outside the geographical service area of the first local telephone exchange without significant custom engineering.
Number portability is the ability to take an existing identification number, e.g. telephone number, assigned by a first location, and reassign it to a second location. The first and the second locations refer to locations of local telephone exchanges, but more broadly to geographical locations and regions. Such number portability is common and popular in mobile networks, and is often referred as full mobile number portability. Number portability is important, and often critical, in the 21st century because as economies advance, mobile phone usage increases, thereby necessitating the need to port numbers from one location to another location. Number portability is also important because it is a tool used by countries and regions to encourage competition amongst service providers by reducing the friction posed to a subscriber as the subscriber contemplates switching service providers.
In traditional number portability schemes, two types of numbers are used: a logical number and a physical number. When a subscriber switches phone service, the logical number is ported to the new service provider. The new service provider, meanwhile, has a new physical number that is corresponding to the logical number. When a telephone call is made to the old physical number, the old physical number is first converted to a logical number, which is used for finding the new physical number in order for locating the geographical and physical locations of the callee. The intelligent platform for converting the physical and the logical numbers has the subscriber information for the callee that is necessary for providing the conversion.
Internet Protocol Multimedia Subsystem (IMS) is a standardized set of architecture specifications of the Third Generation Partnership Project (3GPP) for providing mobile and fixed multimedia services. It uses a voice-over-IP implementation and runs over the Internet Protocol (IP), and is backward compatible with existing phone systems that use packet-switched or circuit-switched technologies. The basic principles of IMS and 3GPP are well known by those skilled in the art and are available at http://www.3gpp.org/specs/numbering.htm.
Because of the way IMS is structured, it is organically flexible enough for the implementation of number portability, i.e. there is no requirement that a physical location be bound by a fixed telephone number, because in the universe of IMS network, there are already a private and a public identification numbers. The public identification number is analogous to the telephone number used in the old PSTN system, which is used publicly for communication sessions. The public identification number is of a Session Initial Protocol (SIP) Universal Resource Identity (URI) or Tel URI format, with Tel URI format used for conventional E.164 numbers. The definitions for Tel URI and SIP URI can be found in RFC3966 and 3261 and 2396.
On the other hand, the private identification number is provided by the network operator of the home network that the user belongs to. The private identification number is the only mechanism for identifying the relation between the subscriber and her home network. By using network access identifier (NAI) scheme, as defined in RFC2486 (http://rfc.net/rfc2486.html, the contents of which are incorporated herein by reference), IMS allows the registration, authentication, management, and billing of a subscriber account to be performed based on the private identification number and independent of the public identification number. As the IMS network uses SIP for the registration process to admit a subscriber into the network, the IMS network relies largely on the private identification number for subscriber authentication and session establishment of a secured channel. Once the registration is completed, the Serving-Call Session Control Function (S-CSCF) of the home network establishes a predetermined relation between the public identification number and the current location of the subscriber. As such, regardless where the location of the subscriber is, either in its home network or any other roaming network, the public identification number can be used to call the subscriber through network routing.
In the IMS network, the S-CSCF needs to process the SIP URI based on its domain information to find out a corresponding Interrogating-CSCF (I-CSCF) that a callee associates with from a domain name and addressing system (DNS). Therefore, it is required that when the network operator provides a public identification number to a subscriber, the domain name of the public identification must be identifiable by the DNS, and the DNS search result must identify the I-CSCF of the domain or network that the operator is operating for the subscriber. As such, the domain portion of the public identification number cannot be dynamically determined, and actually, is pre-fixed. For example, for a user Alice, whose public identification number in a network operated by Operator A may be represented as “sip:alice@operatorA.com”, wherein the domain portion “operatorA.com” cannot be dynamically determined or changed. When a subscriber tries to use a public identification number that is not associated with the domain name of the operator, it is currently impossible to change it.
While IMS network is flexible, it neither mandates nor implements number portability. Furthermore, it suffers from the fact that the old network must be involved extensively in order to forward calls or call sessions. For example, it requires the old network to provide unconditional call forwarding function to the ex-subscriber. This unconditional call forwarding is not only taxing but highly inefficient. Furthermore, in order to facilitate unconditional call forwarding, the old network has to maintain a database of the mapping between the logical number and the physical number
Therefore, desirable in the art of telephone network designs are additional method and system for providing more efficient number portability without taxing the old network, especially for IMS networks.